This invention relates to a microelectromechanical systems (MEMS) device and its method of manufacture. More particularly, this invention relates to a material and process for bonding MEMS wafers with a protective lid wafer.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic batch processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be a cantilevered beam which connects one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable cantilevered beam switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
Because the MEMS devices often have moveable components, such as the cantilevered beam, they typically require protection of the vulnerable moveable portions by sealing the devices under a protective cap or lid wafer, to form an encapsulated MEMS device. Furthermore, the MEMS device may be designed to operate in a particular ambient environment. For example, a MEMS switch handling high voltages may be required to operate in an electrically insulating environment. For this reason, the MEMS switch may be encapsulated with an electrically insulating gas. In order to prevent the preferred gas environment from leaking out over the lifetime of the switch, the environment may need to be sealed hermetically when the MEMS device wafer and the lid wafer are bonded.
The lid wafer may be secured to the device wafer by some adhesive means, such as a low outgassing epoxy. To fabricate the encapsulated MEMS device, a second wafer upon which the MEMS devices have been fabricated is placed against a first, lid wafer. Adhesive may have been placed on the second wafer or the first wafer, or both. The second wafer is pressed against the first wafer, and heat is applied to fuse or cure the adhesive. After curing, the second wafer and first wafer bonded assembly is generally sawed to singulate the individual devices.
Many adhesives such as epoxies, cements and glues are liquid during application, and only harden upon curing. Alternatively, an adhesive such as a solder or metal can be melted until it flows, and then cooled to harden. In either case, the adhesive may need to be a liquid at some point in order to accommodate variations in the surfaces of the first wafer and the second wafer and securely bond the surfaces. The liquid will, in general, flow outward from the bond region during assembly, such that a rigid feature or standoff may need to be provided in the first wafer or second wafer to define a minimum separation between the first wafer and the second wafer. The separation may be that required to accommodate the height of the MEMS device, as well as some additional room to provide a tolerance to allow movement of the MEMS device.
FIG. 1 shows an example of a portion of a prior art first wafer for forming the protective lid for a MEMS device and having a standoff to define the minimum separation between a first wafer and a second wafer. The MEMS device 140, is shown only schematically in FIG. 1, and has been previously formed on the second wafer 150. The first wafer 160 is processed to form a recessed region 170. This recess is sufficiently deep to provide clearance for the MEMS device 140 and its movement. The recess 170 may be formed, for example, by reactive ion etching the surface of the first wafer 160, after appropriate patterning with photoresist. During formation of recess 170, the mechanical standoffs 120 may be formed by protecting these areas from the reactive ion etching process. Alternatively, standoffs 120 may be formed by depositing a material, such as a metal film, in these regions.
The second wafer 150 is generally bonded to the first wafer 160 with an adhesive bond, using a wafer bonding tool. To achieve the adhesive bond, a layer of adhesive 110 is deposited on the cap or first wafer 160, or on the second wafer 150, around the perimeter of the MEMS device 140. The second wafer and first wafer may be aligned so that the standoff features 120 are properly placed with respect to the MEMS devices 140, and clamped together to form the wafer assembly 100. The wafer assembly 100 may then mounted in the wafer bonding tool. The assembly 100 may then be heated to liquefy or cure the adhesive 110. Because of the pressure, the liquid adhesive 110 flows outward from the bond region, allowing the second wafer 150 and the first wafer 160 to come within a minimum distance defined by the standoffs 120. The assembly 100 is allowed to remain stationary until the second wafer 150 is permanently bonded to the first wafer 160. The assembly 100 is then cooled and removed from the wafer bonding tool. The devices are subsequently singulated, to form the individual eancapsulated MEMS devices.
Using the approach illustrated in FIG. 1, the first wafer 160 must be processed to form the standoffs 120 before alignment and bonding to the second wafer 150. This processing may take the form of one or more additional photolithography steps, such as deposition of photoresist, patterning of the photoresist, and followed by etching of the first wafer 160. These additional steps add cost and complexity to the formation of the encapsulated MEMS device.